During travel of an engine-powered vehicle, there are many instances when the vehicle must stop before a destination is reached. This may occur, for example, when the vehicle stops at traffic signals, cross-walks, stop signs and the like. A so-called micro-hybrid vehicle may enable a stop/start strategy for starting and stopping the vehicle engine during a driving event. The engine is shut down if no power is required (e.g. while waiting at a traffic light). As soon as power is requested, the engine is automatically restarted. By avoiding unnecessary engine idling, the vehicle's fuel economy will be improved. For this reason, it is desirable to use the engine shut down function as much as possible when certain engine stop conditions are satisfied.
For a conventional vehicle launch on a hill, a hill-start brake hold function, using a driver-activated brake, may be used to help the driver launch the vehicle on an uphill gradient by preventing the vehicle from rolling backward. The brake hold function is usually activated when the estimated road gradient is higher than a calibrated threshold level; for example about 7% for vehicles with automatic torque converter transmissions. Below this threshold, powertrain creep torque is sufficient to counteract a negative road gradient load on the vehicle such that the vehicle begins to move forward once the driver actuated brake is released.
In the following description, a 7% gradient threshold will be used in describing an example embodiment without loss of generality.
Vehicle torque at zero accelerator pedal input is:
TVeh=TPT+TBrk−TRl, where TBrk is brake torque, TPT is the powertrain torque at wheels and TPT=TCreep when the vehicle is idling. TCreep is creep torque and TRl is road load due to gravity.
Brake torque at vehicle standstill is:
TBrk=TRl−TCreep (assuming sufficient brake pressure input)
Gross torque on the vehicle at vehicle standstill is:TVeh=0
Gross torque on vehicle during vehicle launch after brake release is:TVeh=TPT−TRl and before pressing accelerator pedal:TVeh=TCreepTRl TCreep−TRl>0 when the road gradient is less than say 7%.TCreep−TRl<0 when the road gradient is larger than 7%.
In contrast to a conventional vehicle, a micro-hybrid electric vehicle enables an engine stop-start function at vehicle standstill. It is possible for the vehicle to roll backward downhill after brake release and before the engine is started, where TPT=0, even if the gradient is less than 7% (it is assumed here that the brake hold function will take care of the hill start assistance for gradients higher than 7%). The backward roll is due to the absence of powertrain creep torque during engine stop.                Gross torque on vehicle during launch after brake release, but before engine start, is:        TVeh=−TRl<0, where TVeh is gross torque on the vehicle due to gravity and −TRl is a negative torque component in a direction parallel to the roadway.        
Such a negative torque on the vehicle may cause the vehicle to roll back and even stall the engine during its startup process. The effect of the negative torque also depends on the vehicle load condition. This is demonstrated by the following relationship:TRl=(Mvehicle+Mpayload)g sin(θ),                where θ=road slope angle corresponding to the gradient,Mvehicle=mass of the vehicle.Mpayload=vehicle payload mass.        
The heavier the vehicle is loaded (Mpayload), the higher the negative torque on the vehicle.
It is desirable to hold a micro-hybrid vehicle at a standstill before the engine startup regardless of external conditions. One solution to this problem is to reduce the brake hold gradient threshold down to a lower gradient level, e.g. 3%, such that the vehicle launch process can be supported by the brake hold function to avoid possible vehicle rollback when there is a road gradient. However, the following problems make the use of the brake hold function unfeasible for solving the micro-hybrid vehicle launch problem in a low road gradient range for the following reasons.
1. The brake hold function depends on powertrain torque estimation to determine when to release brake pressure. When the estimated powertrain torque {tilde over (T)}PT is higher than the estimated gradient load {tilde over (T)}Rl, (i.e. TVeh=TPT−TRl>0), the brake hold will start to release brake pressure. If it is not higher, it can maintain the brake pressure close to the initial driver input level such that:TVeh=TPT+TBrk−TRl=0, andTBrk=TRl−TPT>0.
The problem of compensating for the negative road gradient load before the engine startup then would be solved and there will be no vehicle rollback. But, this solution will present another problem. The powertrain torque estimation is far from accurate during an engine startup process. It is only until the engine comes to a steady state that the powertrain torque estimation will converge to the true powertrain torque value. Consequently, if the estimated powertrain torque is too high, early brake pressure release will be commanded by the brake hold function, which may result in unexpected vehicle rollback motion. This could cause the engine to stall. On the other hand, if the estimated powertrain torque is too small, the brake pressure release will be delayed. This delay will drag the vehicle launch attempt as demonstrated by the following equations:TVeh=TPT+TBrk−TRl=0TBrk=TRl−TPT<0.Such a consequence is undesirable.
2. If the vehicle is standing on a near flat surface (e.g., −3%˜3%), the driver does not expect any delay in vehicle launch. However, the road gradient estimation usually does not have good accuracy, especially in a low gradient range. The road gradient estimation accuracy is also affected by environmental conditions (e.g. temperature) and gradient sensor signal quality. As a result, lowering a brake hold gradient threshold may bring a brake hold function induced vehicle launch delay in a flat ground vehicle launch, which sacrifices vehicle performance.
When the vehicle launch is in a low gradient range (e.g., −3%˜7%), the powertrain creep torque is sufficient to overcome the road load torque on the vehicle as the launch of the vehicle is started. In this range of operation, the brake hold function for vehicle launch assistance is expected to be terminated once the vehicle creep torque will be available from the powertrain. The creep torque will move the vehicle forward without advancing the accelerator pedal. The presence of the creep torque, even if there could be a small level of vehicle rollback, which happens in a case of an inaccurate gradient estimation, may cause slight vehicle motion. But that can be sufficiently and promptly controlled by most drivers. Driver intuition will help the driver successfully handle this situation. The brake hold function for hill launch assistance/hill hold control thus would give control authority back to the driver by releasing the brake pressure control back to the driver's input level as soon as powertrain torque will be available and sufficient for vehicle launch purposes.